1. Field of Invention
The present invention relates to a surface light source device used for backlighting an object-to-be-illuminated, and to displays provided with the surface light source device. The present invention is applicable to interior illumination devices or surface light source devices for advertising plate or the likes.
2. Related Art
It is known well to apply a surface light source device to backlighting of an object to be illuminated such as LCD panel. FIG. 19 shows a surface light source device in accordance with of a first prior art. Surface light source device 101 includes light guide plate 104 having a planed shape like a rectangle a corner portion of which is cut off obliquely to provide incidence face 105. LED (point-like light source) 106 is disposed opposite to incidence face 105, light from which enters into light guide plate 104 through incidence face 105 to become an inner propagation light.
The inner propagation light becomes gradually an emission light from emission face 107 of light guide plate 104. The emission light is supplied to LCD panel 103 after transmitting though light control member 114 such as light diffusion sheet. There arises a problem that diverging angle of inner propagation light is not large enough because LED 6 emits light generally having a diverging angle not large enough. This brings an unevenness in emission brightness. In particular, areas near to sides 115, 116 adjacent to incidence face 105 tend to be supplied with insufficient light, being apt to be dark.
FIGS. 20a to 20d illustrate a second prior art employable to overcome such problem, being disclosed in Document 1 noted below. According to the second prior art, light guide plate 104 has incidence face 105 provided with many isosceles-like recesses 117 or prismatic projection rows. Light from LED 106 is much diverged by recesses 117 or prismatic projection rows on entering into light guide plate 104. This causes areas near to sides 115, 116 adjacent to incidence face 105 to be supplied with sufficient light.
However, surface light source device 101 is faced with a problem. In FIG. 20a, a range of effective light emitting surface (effective light emitting area) 126 is a rectangular area denoted by dotted-line. Two sides are denoted by reference numerals 115a and 116a, respectively. Incidence face is formed by cutting off corner portion C1 so that an imaginary line angle-bisecting an angle made by sides 115a and 116a makes the right angle with respect to incidence face 105 (under neglecting unevenness) 105. Optical axis of LED 106 is perpendicular to incidence face 105 (under neglecting unevenness).
It is noted that “optical axis” is defined as a center direction (a travelling direction of the most intensive light) of three-dimensional emission from LED 106. In addition, “effective light emitting area” is defined as an area which is actually used as an emission surface. Usually, a part outside of this area is covered with a frame member.
Emission face 107 of light guide plate 104 can be regarded as being divided into first emission region 107a (out of hatched section) and second emission region 107b (hatched section) by optical axis 1020 of LED 106. Attention is to be paid to a fact that first emission region 107a and second emission region 107b are different from each other in area as understood from FIG. 20a. 
That is, second emission region 107b is an emission region including diagonal 121 extending from a corner portion of effective light emitting area 126, which corresponds to corner portion C1, and is larger than first emission region 107b in area. An inner propagation light in light guide plate 104 has a diverging angle of a range generally symmetric with respect to optical axis 120.
As a result, emission light quantity per unit area of first emission region 107a is larger than that of second emission region 107b. This leads to unbalance in brightness.
A third prior art employable to overcome such unbalance in brightness has been proposed, being disclosed in Document 2 noted below. In FIG. 21, a surface light source device of the third prior art is represented by light guide plate 104.
This light guide plate 104 is featured by an inclination given to incidence face 105 formed at a corner portion thereof. Namely, incidence face 105 is more inclined with respect to side 115 than with respect to side 116.
In other words, balance of brightness between both sides of optical axis 120 is improved by forming incidence face 105 so that optical axis 120 of LED 106 is inclined toward a region providing abundant long light guiding paths of light guide plate 104.
However, if optical axis 120 of LED 106 is merely inclined toward the region providing abundant long light guiding paths of light guide plate 104 as illustrated in FIG. 21, a dark are (hatched area) 119 can appear in the vicinity of one side 116 if light guide plate 104 is shaped like a non-square-rectangle.
A forth prior art employable to overcome such shortage is illustrated in FIGS. 22a to 22f. According to the forth prior art, incidence face 105 is provided with triangle-like recesses 125, each having a pair of slopes 123 and 124 which have inclination angles asymmetrical with respect to a general extending plane of incidence face 105.
It is noted that a general extending plane of an incidence face formed by cutting off a corner portion of light guide plate is called “imaginary incidence face” in the present specification. An imaginary incidence face for incidence face 105 is denoted by dotted line in FIG. 22e. 
In FIG. 22e, light beams 122a, 122b represent light perpendicular to the imaginary incidence face. Light beam 122a is incident to slope 124 having a small inclination angle, becoming light beam 122a′. Light beam 122b is incident to slope 123 having a large inclination angle, becoming light beam 122b′. 
It is to be noted that sufficient light is supplied to around side 116 because light beam 122b′ involves a larger refraction angle as compared with light beam 122a′ (θ1<θ2).
However, this art involves causing optical axis 102 itself to be biased toward the side of side 116 as compared with a case symmetric slopes are formed. As a result, a dark area 127 is apt to appear on effective light emitting area 126 in the vicinity of side 115, one of sides 115, 116 adjacent to incidence face 105. In addition, an excessively bright area 128 can appear on effective light emitting area 126 in the vicinity of the other side 116.
As described above, conventional surface light source devices fail to have enough ability to cause an emission face of light guide plate to provide a uniform emission.                DOCUMENT 1=Tokkai 2003-331628        DOCUMENT 2=Tokkai-Hei 11-133425        